In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler. The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. The reactor vessel is normally constructed of carbon steel and rated for operation at pressures up to about 50 bars (or about 3.1 MPa). Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
Operation of most reactor systems is critically dependent upon good mixing for uniform reactor conditions, heat removal, and effective catalyst. The process must be controllable, and capable of a high production rate. Due in part to the high cost of catalyst and the need to control the rate of reaction, very small amounts of catalyst are used to affect the polymerization of ethylene and co-monomer in gas phase polyethylene production. However, small amounts of impurities in feedstock, even at sub-ppm levels, can adversely affect reactor operations by deactivating the catalyst. Impurities in gaseous feedstocks for polyethylene production typically include H2O, O2, CO, CO2, acids, sulfur compounds and other compounds. Such impurities can impact operations by deactivating catalyst. As catalyst becomes deactivated, the production rate suffers. If high levels of impurities are present, production may cease entirely. While theoretically, the injection of more catalyst into the system would maintain production, it is not desirable to do so. Rather, it would be preferable to identify the source of even minute levels of catalyst-deactivating impurities as soon after their introduction into the reactor system as possible.
Other effects such as static generation have also been attributed to low levels of impurities. In the case of Ziegler-Natta catalysts, the impurities can react with an aluminum alkyl, used as a typical activator or cocatalyst, and form prostatic agents. Electrostatic forces are believed to be a major factor in problematic and frequent “sheeting” events. Sheeting is associated with the undesirable accumulation of polymer along the reactor wall in the zone occupied by the main fluid bed. This accumulation is believed to be associated with fine particles or “fines,” the fines being less than 100-200 mesh. These fines are more influenced by static electrical forces due to their larger surface area relative to their mass, a counter-play of static versus inertial forces.
The stagnation of the resin particles results in a significant reduction in the heat transfer from the nascent particles, precisely at the point in their growth when heat generation per unit surface area is at a maximum. The next result is an interplay of forces which results in particle overheating, melting and agglomerating with adjacent particles, both overheated and normal type particles. The net result is the formation of sheets along the vessel wall. Progressive cycles in this process eventually result in the growth of the sheet and its falling into the fluid bed. These sheets interrupt fluidization, circulation of gas and withdrawal of the product from the reactor, requiring a reactor shutdown for removal.
Background references include U.S. Pat. Nos. 4,855,370, 4,888,948, 5,034,479, U.S. Patent Application Publication No. 2005/148742, and DE 10 2004 019387 (Abstract).
Accordingly, it would be desirable to detect the presence of impurities in gas phase polyolefin and other reactor systems so as to allow avoidance of the problems associated with such impurities.